From the 2009 NBME Bulletin (
http://www.usmle.org/General_Information/bulletin/2009/2009bulletin.pdf):
Special Notice for MD/PhD Candidates
The common pathway for MD/PhD students involves completing the first two years of medical school and then moving to graduate school studies and research for a three- or four-year period. Following completion of PhD course work and all or most of their research projects, these students return to complete their two clinical years, thus completing the medical degree in seven to nine years after first matriculating.
The USMLE program recognizes that the recommended seven-year time limit may pose problems for medical licensure for some candidates with a combined degree (i.e., MD/PhD). It is for this reason that the USMLE program recommends to licensing jurisdictions that they be willing to consider exceptions to the seven-year limit for MD/PhD candidates who meet certain narrow requirements. The recommended requirements are as follows:
1. The candidate has obtained both degrees in an institution or program accredited by the LCME and regional university accrediting body.
2.
The PhD studies should be in a field of biological sciences tested in the Step 1 content. These fields include but are not necessarily limited to anatomy, biochemistry, physiology, microbiology, pharmacology, pathology, genetics, neuroscience, and molecular biology. Fields explicitly not included are business, economics, ethics, history, and other fields not directly related to biological science.
3. Candidates seeking an exception to the seven year rule should be required to present a verifiable and rational explanation for the fact that he or she was unable to meet the seven-year limit. Although these explanations will vary considerably, each licensing jurisdiction will need to decide on its own which explanation justifies an exception.
Students who pursue both degrees should understand that while many states' regulations provide specific exceptions to the seven-year rule for dual degree candidates, others do not. Students pursuing a dual degree are advised to check the state-specific requirements for licensure listed by the FSMB.
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Also - this table provides a listing of some states that provide an explicit exception for MD/PhDs (doesn't mean the other states would necessarily force you to retake):
http://www.fsmb.org/usmle_eliinitial.html